It has been conventional in the petrochemical industry to scrub gases produced by cracking for ethylene production (or by other hydrocarbon conversion processes), with aqueous sodium hydroxide solution. Such a scrubbing treatment removes hydrogen sulfide and carbon dioxide as sodium sulfide, sodium carbonate and sodium bicarbonate and also removes some of the higher molecular weight hydrocarbon constituents carried by the cracked gas stream. Discharge of such effluent would be environmentally harmful and at the very least neutralisation is necessary. However, neutralisation involves regeneration of carbon dioxide and some of the hydrogen sulfide but still leaves a substantial content of hydrogen sulfide and sulfide ions which have a high COD. Processes have therefore been developed to oxidise the sulfide (and other inorganic sulfur acid salts) in the alkaline solution to at least thiosulfate ion and generally to the more environmentally acceptable sulfate ion. Such oxidation processes are generally Wet Air Oxidation (WAO) processes in which gaseous oxygen in the form of fine bubbles is contacted with spent caustic in contacting columns for relatively long periods. The basic WAO process has been known for many years. Such a process which oxidises to thiosulfate is described in Smith, A. G., "Ridding Process Waters and Caustic Solutions of Sulphides", Oil and Gas J, Jul. 9, 1956, p. 95-99. Two articles by Otto Abegg in the publication Erdol und Kohle, Erdgas, Petrochemie (1) Vol. 14, No. 8, p 621-626 (1961) and (2) Vol. 15, No. 9, p 721-722 (1962) describe experiments aimed at complete conversion of sulfide to sulfate in the WAO process. Other descriptions of WAO processes are included for example in Martin, J. D. and Levanas, L. D., "New Column Removes Sulfide with Air", Hydrocarbon Processing & Petroleum Refiner, 41, May 1962, p. 149-153 and in U.S. Pat. No. 3,963,611 assigned to Chevron Research Company and U.S. Pat. No. 3,761,409 assigned to Texaco Inc. Various process variables are described such as temperature, pressure and steam injection as well as gas/liquid contacting devices and co-current and counter-current arrangements with and without catalysts.
Chemical Reagent Oxidation (CRO) of sulfide-containing effluent has also been proposed. For example, U.S. Pat. No. 4,417,986 assigned to Mobil Oil Corporation proposes the use of hydrogen peroxide and U.S. Pat. No. 4,584,107 assigned to Sumitomo Metal Mining Company Limited proposes the use of ozone.
One of the problems which has been encountered with such oxidation treatments and with WAO treatments in particular is the formation of polymeric material which tends to clog lines, contacting towers, valves, etc. and it is postulated that the oxygen used in the oxidation induces polymerisation of at least the more readily polymerisable hydrocarbons carried over with the spent caustic, the oxidation being the first stage at which oxygen is permitted to contact the effluent. The amount of hydrocarbon material is generally small but even if it is large enough to be decanted from the surface of the effluent a certain amount will remain dispersed in the aqueous phase.